1. Field of the Invention
The present invention relates to belt tracking apparatus and, more particularly, to apparatus for tracking an endless belt in a stable, equilibrium lateral position despite changes in belt configuration.
2. Description of the Prior Art
As is well-known, if the longitudinal axis of rotation and the outer surface of a roller are exactly parallel, then a belt or web of material approaching the roller in a direction perpendicular to the axis of rotation will continue to follow the same path of movement about the roller surface until the web is disturbed. However, in normal operation, the outer surface of the roller is not exactly parallel to the axis of rotation and the web does not approach the roller exactly perpendicular to its outer surface. As a result of this misalignment, the web will shift laterally along the roller, tracing a spiral path on the roller surface until it is approaching the roller in a perpendicular manner. Lateral movement of a web along the outer surface of a guide roller can also be caused by uneven tension across the width of the web, web camber or web hydroplaning at high translational speeds.
A simple way to track a web of material along a predetermined path of movement is to provide the rollers with a flange at each end which mechanically forces the web to run between the flanges. However, since the web still tends to travel in the direction it would if the flanges were not there, this tracking technique produces concentrated loading at the edges of the web, resulting in excessive wear and a tendency for the web to climb the flanges, causing deformation and creasing of the web edge.
Another well-known web tracking technique utilizes crown rollers. Such rollers force the web in toward the apex of the roller, resulting in a bulge in the web surface as it conforms to the shape of the crown roller. This tracking technique is, therefore, inappropriate when the web must be guided through the mechanism in a flat condition.
In still another well-known tracking technique, provision is made to detect the position of a lateral edge of the web and to tilt a web steering roller angularly in either of two opposite directions from a normal or neutral position when a deviation is sensed. A disadvantage of this tracking technique is that the web position oscillates with significant amplitude both laterally and normally to the belt plane as the web continuously moves back and forth between two limits. To stabilize the web tracking, a feedback signal representative of the tiltable web steering roller position can be included in the control mechanism, as described in U.S. Pat. No. 2,716,026 to Axworthy. In Axworthy's device, a first signal representative of the lateral position of a web and a second signal representative of a tiltable web steering roller position are compared with a command signal representative of the desired lateral web position. Whenever the magnitude of the sum of the first and second signals deviates from the command signal by a predetermined amount, a control signal activates an electromechanical mechanism which tilts the steering roller so as to direct the web back to the desired tracking position.
However, in the tracking of endless belts or webs of material, additional position-disturbing forces are produced by imperfect belt splicing or by splices in which one end stretches differentially relative to the other and by the introduction into the machine of a new belt having a slightly different configuration. Thus, if the shape of an endless belt were changed in a control system such as in the Axworthy patent, the feedback signal representative of the steering roller position would have a non-zero value at the previous equilibrium tracking position. This means that, with a fixed command signal, the Axworthy device would, within predetermined limits, track an endless belt in a stable equilibrium position, but that such equilibrium position would change with each change in belt shape.
In certain applications, it is desirable to track an endless belt or web of material in the same path of movement regardless of the above described changes in belt configuration. For example, in an automatic electrophotographic copying apparatus wherein an endless photoconductive belt is repeatedly driven past a plurality of processing stations, deviation in belt position adversely affects operations at various stations along the belt path, e.g., the uniform charging pf the photoconductive surface, the proper alignment of the projected original image onto the belt and the subsequent transfer of the developed image onto a support material.